Treating diabetes remains a substantial burden for patients and their families, with up to 50% of patients experiencing devastating secondary complications due to a lifetime of exposure to elevated glucose levels. Currently the only way to restore and sustain insulin without the associated risk of hyper- or hypoglycemia is to replace the patient's insulin-producing cells, the islets of Langerhans: either by the transplant of a vascularized pancreas or by the infusion of isolated islets. However, suitable human pancreas donors are very rare. Pigs provide a potentially unlimited source of islets for xenotransplantation to diabetic patients, and can be developed to the point of clinical applicability, potentially well before other developing technologies, such as stein cells.
The potential for transmission of viruses from the donor to host tissue remains an impediment to the use of xenotransplantation for the treatment of diabetes. Although rigorous biosecurity and testing can eliminate most agents from potential donor pigs, one agent in particular is recalcitrant to this approach. Most, perhaps all, vertebrate genomes, infections. Although most of these are functionally inactive, pig cells contain several types of ACTIVE retroelements called porcine endogenous retroviruses (PERVs). These agents are generally innocuous to the pig, but are of major concern for xenotransplantation. Under laboratory conditions in which human and pig cells are co-cultured, transmission of PERVs from pig to human tissue has been demonstrated (Patience et al. (1997) Nat Med 3, 282-286). It is unclear what, if any ramifications this transmission would have for a patient, but the theorized possibility that PERVs alone or in combination with human agents could cause disease has emerged as a major hurdle to the widespread application of xenotransplantation.